Spark plug heat ranges

Introduction to spark plugs

Example of an NGK spark plug

The heat range (or temperature) is one of the most commonly misunderstood things about spark plugs. It is however fairly straightforward once you have forgotten a few common myths and are aware of the contradictory numbering system used by various manufacturers. So let me try and explain…

Firstly, lets get rid of a common misunderstanding about spark plug heat ranges and say what this does NOT mean. The spark plug heat range does NOT control the temperature at which the engine runs; fitting a ‘cold’ plug will not make an overheating engine run colder, nor will fitting a ‘hot’ plug make a cool-running engine run warmer.

Engine temperature is governed by factors such as the timing and fuel-air mixture settings, running conditions, lubrication quality, compression and cooling system design to name a few, but certainly NOT by the spark plugs heat range. The spark plug gets its heat from the burning fuel in the combustion chamber, and not the other way round!

Hot vs. cold plugs

So what does the spark plug temperature rating actually mean then? Well it is all to do with how well the spark plug dissipates heat from its tip (which is obviously in the middle of the combustion process) through to the cylinder head. A ‘hot’ plug has more insulation and so retains more heat at its tip, whereas a ‘cold’ plug has less insulation and so the tip remains at a lower temperature.

When we talk about more or less insulation, in practice what this typically relates to is the length of the insulator nose. This is the white ceramic part of the spark plug which you see if you look at the end that is normally within the engine. It has the firing tip at one end and the other end is recessed up into the metal casing (the thread part) of the plug.

Comparison of 'hot' and 'cold' spark plugs

We can see this more clearly if we look at the photo on the left which shows two sparks plug side-by-side which have had the threaded section of the metal casing cut away to reveal the full insulator nose.

The one on the left is what we would call a ‘hot’ plug; the tapered section of the white insulator nose is long so that it will only come into thermal contact with the metal body of the spark plug well up into the main body.

The one on the right on the other hand is what we would call a ‘cold’ plug as the tapered section of the insulator nose is much shorter. The straight-sided section of the insulator nose would be in thermal contact with the threaded part of the metal plug body giving a much shorter path for heat to dissipate into the cylinder head.

Optimum spark plug temperature

So now we have seen how hot and cold rated plugs look different, what does this actually mean in practice when we come to select the correct spark plug for our engine? Well like everything else, there is an optimum temperature for the firing tip when the engine is in normal operation. This is a bit of a balancing act between two contradictory requirements, namely carbon fouling and overheating

Example of a fouled spark plug

If a spark plug is too cold then residual deposits from the combustion process will tend to build-up on the end of the plug. After a while these may start to bridge the gap between the firing tip and ground electrode reducing the size of the spark and hence impairing engine performance. Eventually the spark gap may be completely shorted so that no spark occurs at all.

Carbon fouling tends to occur at plug tip temperatures below about 450°C and so the optimum plug temperature is generally above 500°C (932°F). At these temperatures, any accumulation of carbon on the tip will be quickly ‘burnt off’ before it can build up.

Example of a overheated plug

On the other hand, if the tip of the spark plug gets too hot it may cause ‘pre-ignition’ whereby the fuel-air mixture is ignited before the correct moment at which the spark occurs. The glowing spark plug then acts more like a glow plug found in a diesel engine, which is obviously not desirable! Overheating of the spark plug may also cause the electrodes to wear away and the ceramic insulator nose to bubble and breakdown.

To prevent pre-ignition and overheating the optimum spark plug tip temperature generally needs to be below about 800°C (1472°F).

So there we have the optimum temperature range for our spark plug – somewhere between around 500 to 800°C (932°F to 1472°F). This is equally true whether you’re dealing with your classic Matchless motorbike, your car or your lawnmower!

Identifying a plugs heat range

Whilst viewing the insulator nose at the end of a spark plug should give you a good idea or whether a plug is hot or cold, this is not 100% reliable as the majority of the insulator is normally hidden by the threaded metal body of the plug and there may also be internal (thus hidden) differences between plugs. Fortunately though the manufacturers part code will normally tell us the exact heat range, although it is important to note that the notation system used varies between manufacturers and may be completely contradictory! You therefore cannot easily directly compare the heat ranges between different brands. of spark plug.

With NGK spark plugs, the number in the middle of the part code designates the heat range so a “B7ES” has a heat range of 7 and a “BP9ES” a heat range of 9. A smaller number means a hotter plug (2 is the hottest) and conversely a higher number means a cooler plug (12 is the coolest). As a general guide, the difference in tip temperature between otherwise identical spark plugs from one heat range to the next is approximately 70 to 100°C. For more information, have a look at this NGK spark plug reference chart.

When dealing with Bosch spark plugs, the heat range is again designated by the number in the middle of the part code; a “W8DC” has a heat range of 8 and a “W6DC” a heat range of 6. However, the Bosch rating system is the opposite of NGK’s in that a low number represents a colder plug (2 is the coolest) and a higher number a hotter plug (13 is the hottest). This system seems more logical to me, but I’m sure NGK have their reasons for their choice! For further details, have a look at this Bosch spark plug reference chart.

The Champion system is similar to that of Bosch in that the smaller the number, the colder the plug (4 is the coldest) and the bigger the number, the hotter the plug (19 is the hottest). Other manufacturers may have completely different labelling systems again, so it can get very confusing!

Comparison between manufacturers

With all of these different heat range labelling systems it is difficult to compare spark plugs from different manufacturers. However some manufacturers have provided guides which show which of their plugs are equivalent to those of their competitors, which is a great help!

Heat range comparison reference table

I have reproduced some of this data in the heat range comparison table on the left for NGK, Bosch and Champion spark plugs.

Note though that this table should be used as a guide only as some stated ‘equivalents’ may not be exactly the same between different manufacturers.

However, from the point of view of classic bikes, we are not dealing with particularly high-tech or high-performance engines and so this broad comparison is probably going to be more than close enough!

Changing spark plug types

During development, the manufacturer will go to great lengths to determine the most appropriate spark plug heat range for a new engine. So why would you want to change this you may ask? Well most of the time you probably wouldn’t need to, but there are a few occasions when (for the average classic biker) this might be an advantage.

The first thing to note is that manufacturers will always err on the side of caution when designing any aspect of an average car or motorbike and so longevity and reliability will normally always come before obtaining peak performance. Also, vehicles need to be designed to run in different climates, on varying grades of fuel, with different owners who have different driving/riding styles (for example slow town traffic versus lots of high-speed motorway cruising) and with varying levels of maintenance. So engine manufacturers will normally opt for the ‘safe’ option which will give the best reliability for the majority of users.

But you may not be an average user! Perhaps the most common reason to want to change the spark plug type used in a classic bike is that these are generally used less frequently and for shorter, slow-speed runs around town. Therefore the spark plug may not be getting up to the temperatures required to burn off carbon deposits on the tip resulting in frequent fouling. Whilst not a major issue, having to keep cleaning or changing the plug can be a real hassle. So it may be that selecting a slightly ‘hotter’ spark plug might help to reduce fouling.

Also, if you make any significant adjustments to the running of the engine (e.g. different fuel types, additives, timing or mixture) then these may affect the temperatures in the combustion chamber and hence a spark plug with a different heat range may be more appropriate. The key though is not to make more than one change at a time otherwise you’ll never be sure which of these is impacting of the running or performance of the engine.

It is also important to remember that it is better to err on the side of a spark plug which is too cold, rather than one which is too hot. At worst, a plug which is too cold will result in fouling which can easily be remedied with a quick change, but a plug which is too hot could cause severe engine damage.

Conclusions

Well, that’s about it! Not rocket science at all but nevertheless sometimes confusing, especially with all the contradictory numbering systems used by different manufacturers. I hope this brief guide has helped explain things though!

If, like me, you are the proud owner of a Matchless G3Ls motorbike then you will find further useful information about spark plug selection in my ‘Which spark plug?‘ guide.

A friendly disclaimer in case you do something silly!

This article and website are based upon my own opinions, experiences, mistakes and successes and is provided for general interest and information purposes only. I hope you find it useful, but please do not rely blindly on anything I have written or done myself. You and your bike are different to me and my bike and so other considerations may apply. Any damage you inadvertently do to yourself or your bike is solely your own fault; I cannot be held responsible for your actions! The full terms and conditions for my website can be found here.

James, I have a mazda 2004 Tribute that had 145, 000 miles when I decided to replace the plugs with new Iridium tipped. The car ran great before and after replacing the plugs. But four months later my original catalitic converter internally melted and clogged the secondary converter causing a overheated and eventually blown EGR valve. Do you think the Iridium plugs had anything to do with this or was this just long term decomposition of the converter over 145,000 miles? Thanks… Joel

Thanks for your interest in my web site. I can’t help much with your problem I’m afraid – my interest is more in old motorbikes than newer cars. However, generally speaking, the iridium spark plugs seem to be a straight swap for the standard types and I would be surprised if they had caused any problems with you cat. Sounds likes just coincidence that the issues occurred after you replaced the spark plugs, but probably worth speaking with a Mazda expert.

Loved your article, very simple and easily comprehensible. Anyway, I am equally confused about plugs esp when the car is in a different country. I just brought in a classic 68 impala V8 350 from Ohio, USA into my very hot tropical country Malaysia. Noticed that the plugs are Bosch Platinum heat range 9. I intend to change them to probably a 7 or 8 but can’t seem to find them easily even in US. Reason, temp here can easily kill an engine if you don’t have at least 3 electric fans, so now you know why a range 9 plug is not suitable here.

I can’t offer much advice regarding the correct spark plugs for your Impala I’m afraid, my knowledge is really only for my old Matchless motorbike. A Bosch ‘9’ plug is quite a hot one, but remember that the spark plug heat range does not affect the engine temperature; it is just the engine temperature at which the spark plug works best. Therefore a hot plug will not necessarily be bad for your car in a hot country.

I’d suggest speaking to someone who knows about these cars (a dealer, restorer, owners club, etc) to see what plugs they recommend as alternatives, then see which of those you can get locally. The high humidity (I’m assuming it’s quite humid where you are in Malaysia!) will also affect the carburation, so that’s something to watch out for when tuning your car. My bike needs quite different carb set-up here in humid Hong Kong to it did back in dry India, or others use in the US or UK. Just a thought…

A ‘hotter’ spark plug should tend to foul less as the deposits should get burned off quicker, or not form in the first place. But if you go too hot you might have pre-ignition and maybe end up with a hole in the top of your piston which is never good!

Is the mixture set correctly at the carburettor? I’m guessing that this will need to have be re-jetted from standard when the engine was opened up to 1000cc? You really need to get the mixture set correctly first, then double check ignition timing, before starting to play around with spark plug types. Otherwise you’ll be disguising the symptoms rather than fixing the cause.

Nowhere have I ever read (from a spark plug manufacturer) that hotter spark plugs increase cylinder head temp. I thought that they couldn’t because ceramic (the insulator) is a very poor thermal conductor. If I was right about the ceramic then the small area of the center electrode wouldn’t be enough to absorb a lot of the heat. But I was wrong. The ceramic is not a typical kind of ceramic and does conduct a lot of the heat to the body of the plug and to the head. Here are some tests I just did:

55cc ported for 8300 rpm. 18mm Mikuni with reed valve. non-squish-band slant plug head with cranking pressure of 140psi and spark plug screwing in at the rearward intake side of the combustion dome. Jaguar torque pipe. Fiber head gasket insulating the head from the cylinder. Fiber washer insulating the thermocouple from the cooler cylinder studs/nut. Thermocouple connected under a forward head stud/nut leading to digital readout at handlebars.

NGK___max temp
B5HS__299F
B6HS__268F
—————
Second test:
Same engine but with squish band head with 165psi. Metal head gasket. Champion plugs tested which spanned 3 NGK heat ranges. The #87 had an extended tip (protruding more into the combustion area).

Hi Michael. Interesting tests, thanks for sharing. The ‘insulator’ section of the plug is a good conductor of heat (it’s obviously a good electrical insulator though!) and there is more of it in a ‘hotter’ plug so that more heat is transferred to the cylinder head in order to keep the plug tip temperature in the correct range. So I guess this explains the higher cylinder head temperatures you’ve measured on your 2-stroke. But do you think this is having any actual effect on temperatures inside the engine? My guess is probably not. Regards, James

yeah, I wouldn’t worry about temperature increase unless you are already at the limit which is 450F for constant running, 500F short spurts. Higher temps may be desirable even, since the spark jumps more readily with a hotter tip.

Best to buy a digital temperature gauge with the washer thermocouple under the spark plug so you know what is really happening there.
My engine is air cooled too. Water cooled engines may need to run colder plugs since the max they want is 260 degrees (with a 15 lb radiator cap) to keep from turning all their coolant to steam.

Hi Marcus. No, a hotter spark plug should not make any difference to starting the engine. All plugs are cold when the engine is cold, it’s only when the engine is up to temp that the heat rating will make a difference.

Probably a dodgy spark plug, sometimes even new ones might not work. Can you see a spark with the new plug when you take it out, hold it (carefully!) against the head and turn the engine over? Best try another plug! James 🙂

My whipper snipper and chain saw on a long extension suddenly gave up about the same time. I took the plug out of the mower, and away they went.
This caused me to critically examine the plugs. One had the electrode off centre, quite noticeably, so it appears manufacturing problem. The other had a cracked insulator. The thing I noticed was one was a hot plug, the other cold, and as both motors seem almost identical (both cheap China jobs), the only noticeable difference was two stroke mixture. One was 25:1 the other 30:1. Would it be fair to say the extra oil in 25:1 requires a hotter plug to burn the extra drop of oil, or would there be another reason.

Hi Ian. Maybe its because of the oil mixture, but not necessarily. Whilst the engines may appear identical, they could be setup differently (e.g. different operating speeds, carburation, load, ignition timing, etc). Best to stick to what the manufacturer recommends, although a heat range either way probably won’t make much difference. James

James – that was a really good read, however I’d like to add something that could help many who are posting. When questioning the correct heat range in a spark plug you should drive whatever it is at the highest power level you normally run it, kill the engine, stop the engine & remove the plug to look at the color of the insulator. It’s ok to let it cool off before pulling the plug/s. Anything in the browns is ok. With the Ethanol crap served at most pumps now, it’s not uncommon to find a white insulator with clean electrodes – still ok, but getting close to the limit. Any blistering means it’s time for a cooler plug!
Just my .02

I have a 1926 T Ford my plan is to use a modern sparkplug by machining a adaptor/ insert to fit the old head and reduce the hole size and fit a spark plug that I can get from any spare shop. I need to find out what the heat range is of the current plugs that I am using now. By making an adaptor I need to use a long reach 14 x 1.25 mm plug. Can you match an NGK to replace the old type Champion 25 ???????

Hi Hansie. Sounds like an interesting project! I’ll email you a copy of the Champion 1927-55 plug catalogue. It’s a year too late for your Ford and I don’t think mentions a type 25 plug (only types 10, 15 and 17), but maybe it will give you some clues. Regards, James

Hi James,
I am concerned whether ai understand this correctly? I have a 1980 Honda CB750K. The manual says to use NGK D8EA plugs. I found them fouling within 500km of use. I do a lot of city traffic so no high speeds. I changed to a D9EA plug. What do you think? Will cooler help? I do go on road trips on weekends where travelling speed is on average 120km/h.

Hi Kobus. I think you might have this the wrong way around! To reduce fouling, you will need to switch to a hotter spark plug (i.e. a smaller heat range number for NGK plugs). A hotter plug will run hotter at the same engine conditions compared to your existing plug, hence should burn of any fouling.

Of course this really does depend upon whether your plugs are fouling because of the low speed riding, or because the engine is not running properly in some way (e.g. piston rings leaking oil, carb too rich, timing wrong). Changing the plug won’t fix incorrect engine running!

Changing the plug to one heat range hotter should be safe for normal riding, but you may wish to swap back to the standard plug on longer higher speed road trips. A plug which is too hot could burn a hole in the top of your piston if you’re not careful!

I have a Honda 400RR NC29 from 1994. I faced spark plug issues constantly. They always fouled up if not used for 3-5 days. If bike was unused for weeks I have to clean plugs or replace them for the bike to start. It turns out the fuel mixture was too rich. I got the carbs rejetted. Rebuilt carbs etc. balanced it several times and tried it over a period of 2 years. Same problem persisted. So as a last resort I went one up to a hotter 7 Series NGK plug instead of the recommended 8 series which I was using all these years. Bike has been working trouble free ever since 🙂

Hi Again James, Having now found and read your other articles (should’ve looked first) I think I’ve managed to spec a Bosch H6BC for my 1937 Royal Enfield Model S.
Thanks for all the info that you’ve posted.
Stu

Hi James, Great stuff! My 1969 R69US BMW just came back from a 100% dealer rebuild. The spark plug holes were sleeved with steel inserts. The motor sputters and backfires a lot and the dealer can’t fix it.The plug tips look a very light gray with no fouling. I’m wondering if the sleeves change the plug heat and or may cause pre ignition themselves.Any thoughts will be greatly appreciated. Thanks, Larry

Mr. James. Thanks for enlightening me on the issue of which spark plug to chose. However I am not able to hold my self back from complaining that you have not said a word about 2 stoke bikes particularly Jawa/Yezdi.which was considred the queen of road and a craze in India.And even now fans are rstoring the old classic into a 12 volt system
Presently I am using a Bosch WQ7BC4 Spark plug in my 1979 model B Yezdi The recommended spa?k plug is W225Z of Bosch, which is not available atleast easily. Many heat ange are available like W175Z1,W5BC. WHAT I AM TRYING TO ASK IS WHICH PLUG SHOULD I USE. SINCE MANY OF THEM GOT SHORT BECAUSE OF SOOTING. I AM 65 YEARS OLD AND I TAKE MY BIKE ON LONG RIDES ATLEAST TWICE A YEAR COVERING OVER A MINIMUM OF 800 KM TO AND FRO.PLEASE ADVISE.